1. Field of the Invention
This invention relates generally to a tripping device used in a circuit breaker and more particularly to a tripping device used in a circuit breaker designed to carry high load currents.
2. Description of the Prior Art
Tripping devices are essential to the operation of breakers. Typically, circuit breakers are held closed by a latch and are tripped open by a tripping device acting on a trigger to release the latch. In large circuit breakers and important installations a tripping coil actuated by a control means is used as a tripping device. Generally, a tripping coil includes an armature, an actuator rod, a return spring, a permanent magnet and a d.c. (direct current) coil having a single winding. The permanent magnet biases the armature and actuator rod against the return spring. When the control means energizes the d.c. coil, due to a current overload, the armature and actuator rod are pulled away from the permanent magnet and the actuator rod triggers the release of the latch, thereby causing the breaker to open.
One problem with the tripping coil described above is the spurious tripping of a circuit breaker caused by the magnetic field produced by a high alternating load current (e.g. 85,000 amperes) carried by the circuit breaker. Normally, the tripping coil for a circuit breaker is located in close proximatry to the load current in the circuit breaker. As the current of the alternating load current increases, the flux density in the space surrounding the load current conductors increases. When the load current becomes high enough, the spurious magnetic flux produced can pull the armature and actuating rod away from the permanent magnet, thereby tripping the circuit breaker.
One approach to this spurious tripping problem is the shielding of the d.c. coil in the tripping device from the magnetic flux produced by the load current. This approach has the drawbacks of increasing the size of the tripping coil and increasing its manufacturing cost . In addition, the use of shielding may be ineffective or impractical when the normal load currents in a circuit breaker are very high.
Another approach to this problem is to use either a stronger permanent magnetic or a weaker return spring. This has the drawback of requiring a higher tripping force and correspondingly requires a larger d.c. coil. The use of a larger d.c. coil increases the time it takes the tripping coil to trip the circuit breaker, requires a larger power supply for energizing the d.c. coil, and increases the cost and size of the tripping device.